Today, optical-based data storage systems are commercially competitive due to their high storage density, relatively low cost, and random access capability. Moreover, magneto-optical data storage systems offer the added flexibility of allowing an optical medium to be erased and new data written in place of the erased section. This feature grants a user the capability to reuse an optical medium many times over by erasing old data and substituting new data in place thereof.
Basically, magneto-optical recording operates in the following manner. Data are stored as a series of binary bits (i.e., 1s and 0s). A laser beam is focused onto an optical medium, usually by means of a lens assembly. Initially, the optical medium is perpendicularly magnetized in one direction. To write a "1," the laser beam is pulsed at a high power for a short duration. This raises the temperature of the optical medium to such a degree that an externally applied magnetic field reverses the direction of magnetization in the heated region. When the medium returns to its lower ambient temperature, the "domain" retains its reversed magnetization.
The domains are "erased" by using the laser to perform the same thermal process used to write the data, except that an oppositely directed external magnetic field is applied. Thereby, the domains revert back to their original magnetization.
The stored data are read from the optical medium based on the Kerr effect. This principle states that linearly polarized light, reflected from a perpendicularly magnetized medium, is rotated according to the direction of magnetization. Hence, the magnetization transitions of the domains stored on the medium can be read by determining the direction of the plane of polarization of the reflected light. The same laser used to write the data is also used to generate the reflected light for reading the stored data, except that its power is reduced to avoid inadvertently writing data onto the medium.
An encoding scheme is implemented in order to map the data into a pattern which is compatible with recording media data disks. In other words, the encoding scheme enables the magneto-optical disk drive system to physically distinguish between each of the bits stored on the track. A popular encoding scheme is 2,7 encoding. The 2,7 encoding scheme specifies that there should be a minimum of two and a maximum of seven "0s" between each pair of "1s." Hence, the spatial frequency of the domains depend upon the particular sequence of digital data written the medium.
The magneto-optical disk drive converts these domains into an electrical signal, wherein each read back domain produces a corresponding electrical pulse. As the domains are written closer together, the amplitude of the corresponding electrical signals starts to diminish due to intersymbol interference. As a result, the amplitude of the read signal is proportional to the spatial frequency of the domains. The higher the spatial frequency, the lower the corresponding electrical signal's amplitude becomes.
FIG. 1 is a block diagram of a prior art read channel of an magneto-optical disk drive system. A laser beam is directed onto track 101 of an optical disk by lens assembly 108. The laser beam is modulated and reflected back to detector 109, which converts the beam into an electrical signal 110. Note that the amplitudes for pulses 112-114 corresponding to domains 102-104 are greater than the amplitudes of pulses 115-117 corresponding to the more closely spaced domains 105-107.
The electrical signal 110 is then amplified/attenuated by an automatic gain control circuit 118. The signal is then filtered, and the high frequency components are equalized by block 119. Thereafter, the peaks of each of the pulses 112-117 are detected by taking the derivative of signal 110 and determining the zero crossings of the negative slopes. These functions are performed by differentiator 120 and zero crossing detector 122. The output from differentiator 120, given an input signal 110, is shown as derivative signal 130. Note that the zero crossings 132-137 correspond to the peaks of pulses 112-117. The derivative signal 130 is fed back through full wave rectifier 123, peak detector 124, and AGC control 125 to the AGC amplifier 118.
The pulses represented by derivative signal 130 from differentiator 120 are also qualified by requiring that they exceed a certain threshold level by the threshold qualifier block 121. This is implemented in order to prevent baseline noise such as media noise, laser noise, etc., from falsely triggering zero crossings.
One problem with the typical prior art read channel described above arises from the fact that in optical recording the signal read from the optical media contains DC components. Further complicating matters is the fact that the various stages of the read channel are typically AC coupled in order to achieve high bandwidth. This AC coupling causes the average voltage level to change, depending on the DC component. Thus, the average signal level is varying according to the read pattern. The read pattern is a function of the user data, encoding scheme, etc. To get around this problem, differentiation is performed prior to threshold qualification thereby removing the DC content of the signal. However, differentiation introduces additional noise to the system. In particular, the differentiation boosts the high frequency noise. This may lead to false or missed qualifications of zero crossings. In addition, equalization produces sidelobes upon differentiation. These sidelobes tend to reduce the noise margin, thereby rendering the read channel more susceptible to errors due to noise. One cannot simply restore the DC level to some baseline value because the higher frequency components of the read signal do not return to the baseline due to intersymbol interference. These problems are becoming even more acute as areal densities continue to increase.
Thus, there is a need in the prior art for a more robust threshold and qualification scheme. It would be preferable if such a scheme could efficiently and reliably restore the DC level to the signal from the optical disk. The restored signal could then be used to establish a threshold which could then be used in conjunction with the restored signal to qualify pulses.